The correct answer is D. Copper.

Trace elements (or micronutrients) are essential for plant growth but are required in very small amounts. Copper (Cu) is a trace element that plays a crucial role in various enzymatic processes, photosynthesis, and plant metabolism.

The other options:
- Sodium (Na): Not an essential nutrient for most plants.
- Phosphate (PO₄³⁻): A macronutrient, essential for energy transfer (ATP) and nucleic acids.
- Potassium (K): A macronutrient, important for enzyme activation and water regulation.
- Nitrate (NO₃⁻): A macronutrient, essential for protein synthesis.

Thus, Copper (Cu) is the correct answer as it is a trace element.